(1) Field of the Invention
The present invention relates to a method of manufacturing a hydrogen-absorbing alloy for use in an alkali storage cell, and more specifically to a method of acid-treating the hydrogen-absorbing alloy and an improvement in a treatment which follows the acid treatment.
(2) Description of the Prior Art
In a nickel-hydrogen storage cell where a hydrogen-absorbing alloy is used as a negative electrode active material, the cell performance is affected by the activity of the alloy. For this reason, the hydrogen-absorbing alloy used in this type of storage cell is pulverized into fine powder so as to expand an area concerned with an electrochemical reaction and to enhance the filling density of the electrode substrate, thereby enhancing energy density.
However, a hydrogen-absorbing alloy is so active that it is oxidized during pulverization or in storage, forming an oxidation coating over its surface. This oxidation coating deteriorates the electric conductivity and electrochemical reactivity of the alloy.
To solve these problems, various methods of restoring the electrochemical activity of a hydrogen-absorbing alloy have been suggested. One of such methods is an acid treatment disclosed, for example, in Japanese Laid-open Patent Application Nos. 4-179055, 7-73878, and 7-153460 where the surface of a hydrogen-absorbing alloy is treated with an acid aqueous solution. This acid treatment is characterized by its simple operation and excellent effects of removing an oxidation coating, so that the application of this treatment can facilitate the restoration of the electrochemical activity of a hydrogen-absorbing alloy.
However, even this treatment fails to sufficiently enhance the high-rate discharge characteristic, cell internal pressure characteristic, and cycle characteristic, so that a further improvement has been expected.